1. Field of the Invention
The present invention concerns a method for allocating a timeslot within a frame to a mobile entering a communication cell.
It also concerns a base transceiver station implementing this method.
2. Description of the Prior Art
In a time division multiple access (TDMA) mobile radio network a plurality of transmitters (eight in the case of the GSM system) use the same radio resource by allocating each mobile a timeslot.
In the case of the GSM system (Global System for Mobile communications) the timeslot duration is 577 .mu.s and the frame duration is 4.6 ms.
In most mobile radio systems some frequencies are transmitted continuously to enable the mobiles to identify them more easily given that, during a call, they have only a very short timeslot, as already indicated: for them to make reliable measurements, the power transmitted at these frequencies must always be the same.
In the case of a GSM system using a TDMA structure, timeslot 0 is used on the beacon channel to transmit specific information:
in the downlink direction, it is used to indicate general parameters of the cell to all mobiles, to call them and to allocate them a radio channel; PA0 in the uplink direction it is used by the mobiles to indicate that they require a connection to the cell (random access signal). PA0 listen out for its timeslot at the frequency f1; PA0 transmit in its timeslot at the frequency f2; and PA0 measure the power level at a frequency f3 to determine the level at which it receives from neighbouring cells. PA0 1.5 TS from f2 to f3 and from f3 to f1; PA0 2 TS-TA from f1 to f2. PA0 a timing advance TA=0 can be indicated to the mobile at all times and the receive window moved according to the actual distance between the base transceiver station and the mobile; in this case the temporal structure can handle only four mobiles, each mobile always requiring a pair of timeslots; PA0 the mobile can be managed in the conventional way at distances up to 35 km and a timing advance TA equivalent to 35 km always indicated beyond that distance; the management procedure is more difficult, but enables use of only two timeslots when this is really necessary. PA0 the analog transmit and receive equipments are used to only 50% of their initial capacity, which is all the greater a penalty in the case of the transmitters as they must have a high power rating, given the distances involved; PA0 the digital processing equipment suffers from the same underuse, unless specific and difficult precautions are implemented; this limitation continues to apply to timeslot 0 of the beacon channel, regardless of the implementation selected, because of the problem of the random access signal.
One problem with this type of temporal structure is that the radio signal propagation time cannot be ignored if the mobile is at a great distance from the relevant base transceiver station (BTS).
In current GSM systems this problem is solved in the following manner: when a mobile enters the cell for the first time it sends a random access signal with a time-delay strictly equal to three timeslots; the reason for using this (fixed) value is explained below. When the BTS receives this signal it measures the time difference between it and its own receive timing reference which is offset exactly three timeslots relative to its transmission. This arrival time is equivalent to the return journey time for a radio wave between the BTS and the mobile. The base transceiver station then allocates a timeslot to the mobile and indicates to the latter the timing advance it is to use for transmitting.
Referring to FIG. 1, the mobile MS1 21 in the immediate vicinity of the base transceiver station 20 sends a random access signal RA1 1 with a null time-delay 2 and is allocated timeslot 6 (3). It then transmits with a time-delay equal to 3TS-0, the value 0 being indicated to it when it is allocated timeslot 6.
Likewise, the mobile MS2 22 at a great distance from the base transceiver station 20 sends a Random Access signal RA2 2 with a time-delay TA2 and is allocated timeslot 1 (13). It then transmits with a time-delay equal to 3TS-TA2, the value TA2 being indicated to it when it is allocated timeslot 1.
The choice of the receive/transmit time-delay by the mobile is dictated by the fact that with eight timeslots the mobile must:
The most economic solution for operating at these three frequencies is to use a single frequency synthesizer. As direct synthesis type synthesizers are too costly for a system in which the price of the terminal is a key factor in commercial success, the most appropriate solution is a phase-locked loop synthesizer. As these synthesizers have a relatively long synchronization time it is important to distribute optimally the three timeslots mentioned above and during which the generated frequency has to be stable.
Considering that the mobile measures the power level at the frequency f3 during a timeslot, the mobile has (8-3)/3=1.7 timeslots to change frequency. As the mobile must be able to advance its transmission by a timing advance TA, the decision has been taken to separate the receive and transmit timeslots by a time equal to 2 TS.
The remaining time for switching from one frequency to the other is therefore:
By choosing a maximum value of TA equal to 0.5 TS, the timing constraint for changing from one frequency to another is always 1.5 TS. Other constraints have led to the choice of a maximum value of TA equal to 63 bits of on air modulation, which is equivalent to 0.41 TS. One of these constraints is that the random access signals must always be received in their entirety during the timeslot, regardless of the distance to the mobile: the farther away the mobile, the less the information carrying capacity of the random access signal.
This outline of the reasons which led to this choice clearly shows that it is not possible to increase indefinitely the maximum value of TA for mobiles at increasing distances: the current limit on TA cannot be exceeded without calling into question the electronic design of the mobile and the usable length of the random access message.
For true mobiles the economic benefits of large area cells are relatively minor as the transmit power requirements quickly render the mobiles very bulky. On the other hand, the economic benefits are strong for "fake" mobiles which are actually at a fixed location with a directional antenna pointing towards the base transceiver station, providing radio communications at an infrastructure cost which bears no comparison with that for a cable network. The only limitation is then that imposed by the distance to the horizon.
To be able to exchange data with mobiles at distances in excess of 35 km the solution usually recommended is relatively simple, as explained in "The GSM System" written and published by Michel MOULY and Marie-Bernadette PAUTET (ISBN 2-9507190-0-7); the receive timeslots are grouped in pairs: 0+1, 2+3, 4+5 and 6+7, and the base transceiver station uses a receive window that varies with the distance to the mobile, in one of a variety of feasible implementations:
In all cases timeslots 0 and 1 of the beacon channel used to receive the random access signals have to be reserved for this use only, the random access signals from distant mobiles overlapping these two timeslots.
The overall balance of a system of this kind is rather poor, as compared with a conventional base transceiver station:
A result of all these considerations is that the frequencies are largely underused. This is of less importance as frequency requirements are a priori limited if very large cells are used and as the possibilities for frequency re-use are much greater than with small cells. This is because, as the propagation laws are inverse square laws in respect of the distance, large cells enable the same frequencies to be re-used more often.